The world is a three-dimensional (3-D) place and the human brain is very effective at interpreting data when it is presented in a 3-D format. The human brain interprets a scene as being three-dimensional by a variety of visual clues from which it can extract information for depth perception. There have been many attempts to find an effective way of creating displays that take advantage of visual clues to provide a true 3-D window on the world with practical real-time motion.
The visual clues can be broken down to four main categories which may be replicated by a 3-D display technology. Although an individual eye only sees a two dimensional (2-D) image created on the retina, the observer's brain works with the slightly different views from the two eyes to construct the third dimension of depth from which the brain can create a 3-D image. This process is called stereopsis. To achieve this result, a display must somehow channel a different view into each eye.
Another category is motion parallax, which relies on the relative apparent motion of objects in the field of view as the observer's head moves vertically or laterally. To achieve this result, a display typically gauges and responds to the observer's head motion.
The two other categories provide weaker visual clues. Using eye focusing and convergence feedback, the brain can obtain some information about the depth position of an object by sensing how strongly the eye lens is squeezed to bring the object into focus and the relative angles of the two eyes needed to make the object views converge within the brain. Because, eye focusing and convergence are weak visual clues, most 3-D display systems can operate with the observer focusing at a fixed distance, such as infinity or on a screen close to the observer.
In addition, software-based visual illusions can create weak visual clues within an image generator for display on a 2-D surface. These illusions include object perspective and shading, distance haze and movement parallax from camera motion. Software-based visual illusions are the easiest category of visual clues to produce. Indeed, most simulator and computer games produce these visual illusions by using 3-D worlds within the computer memory and processor before rendering on a 2-D surface.
One example of a three-dimensional display taking advantage of stereopsis includes a movable vertical slit in combination with an image source. The slit is moved between a plurality of positions in sequence with images displayed by the image source. The images can be viewed by a viewer with each image displaced in space based on the position of the slit.
An example of a 3-D display device which can provide both stereopsis and motion parallax is a head-mounted display (HMD) device. Miniature display screens within the HMD device are registered to and provide a respective image to each eye. A head tracking device can be combined with the HMD device to provide lookaround capability with observer head motion. Such systems are, however, limited to use by a single observer at any one time and transport delays can result in nausea and loss of equilibrium.